With the advances in microscopic manufacturing technology, much attention has been focused on so-called micro-machine (MEMS: Micro Electro Mechanical Systems, ultra-miniature electric, mechanical compound) devices and miniature devices in which MEMS devices are incorporated.
A MEMS device is a device that is formed on a substrate such as a silicon substrate, glass substrate or the like as a microscopic structure, and electrically and further, mechanically unites a driving body outputting mechanical driving force with a semiconductor integrated circuit or the like that controls the mechanical body. A basic feature of the MEMS device is that a mechanically structured driving body is incorporated in a part of the device, and the driving body is electrically driven by the use of coulombic attraction force between electrodes or the like.
FIGS. 11 and 12 show a typical composition of an optical MEMS device that is applied to an optical switch and a light modulation device by taking advantage of the reflection or diffraction of light.
An optical MEMS device 1 shown in FIG. 11 includes a substrate 2, a substrate side electrode 3 formed on the substrate 2, a beam 6 having a driving side electrode 4 that is disposed in parallel to oppose the substrate side electrode 3, and a support part 7 for supporting one end of the beam 6. The beam 6 and substrate side electrode 3 are electrically insulated by a void 8 therebetween.
A required substrate such as a substrate with an insulation film formed on a semiconductor substrate of, for example, silicon (Si), gallium arsenic (GaAs) and the like, or an insulative substrate such as a glass substrate is used for the substrate 2. The substrate side electrode 3 is formed of a polycrystalline silicon film by doping impurities therein, metal film (W deposited film, for example), and the like. The beam 6 is composed of, for example, an insulation film 5 such as silicon nitride film (SiN film) or the like and the driving side film 4 serving as a reflective film consisting of, for example, Al film of 100 nm or so in thickness. The beam 6 is formed in a so-called cantilever fashion with its one end supported by the support part 7.
In the optical MEMS device 1, the beam 6 displaces itself in response to electrostatic attraction force or electrostatic repulsion force generated between the substrate side electrode 3 and driving side electrode 4 by an electric potential applied to the substrate side electrode 3 and driving side electrode 4, and as shown by a solid line as well as a broken line in FIG. 11, for example, the beam 6 displaces itself into a parallel state or inclined state relative to the substrate side electrode 3.
An optical MEMS device 11 shown in FIG. 12 is composed of a substrate 12, a substrate side electrode 13 formed on the substrate 12 and a beam 14 that straddles the substrate side electrode 13 in a bridge-like fashion. The beam 14 and substrate side electrode 13 are insulated by a void 10 therebetween.
The beam 14 is composed of a bridge member 15 of, for example, a SiN film that rises up from the substrate 12 and straddles a substrate side electrode 13 in a bridge-like fashion and a driving side electrode 16 of, for example, an Al film of 100 nm or so in thickness, that, serving as a refection film, is provided on the bridge member 15 to oppose the substrate side electrode 13 in parallel to each other. The substrate 12, substrate side electrode 13, beam 14 and the like may employ similar compositions and materials to those explained in FIG. 11. The beam 14 is formed in a so-called bridge-like fashion in which the both ends thereof are supported.
In the optical MEMS device 11, the beam 14 displaces itself in response to electrostatic attraction force or electrostatic repulsion force generated between the substrate side electrode 13 and driving side electrode 16 by an electric potential that is applied to the substrate side electrode 13 and driving side electrode 16, and as shown by a solid line and a broken line as well in FIG. 12, for example, the beam 14 displaces itself into a parallel state or fallen state relative to the substrate side electrode 13.
With these optical MEMS devices 1, 11, light is irradiated on the surfaces of the driving side electrodes 4, 16 serving as a light reflective film, and by taking advantage of differences in the direction of reflected light depending on positions into which the beam 6, 14 are driven, these MEMS devices can be applied to an optical switch having a switch function by detecting the reflected light of one direction.
Further, the optical MEMS devices 1, 11 are applicable as a light modulation device for modulating the strength of light. When light reflection is taken advantage of, the strength of light is modulated by vibrating the beams 6, 14 according to the amount of reflected light of one direction per unit time. This light modulation device runs on a so-called time modulation.
When light diffraction is taken advantage of, a light modulation device is composed of a plurality of beams 6, 14 disposed in parallel relative to the common substrate side electrodes 3, 13, and by varying the height of, for example, driving side electrodes serving as a light reflective film with the movements of every other beam 6, 14 such as moving closer to or moving away from the common substrate side electrodes 3, 13, the strength of reflected light from the driving side electrodes is modulated by means of light diffraction. This light modulation device runs on a so-called space modulation.
FIG. 13 shows a composition of the GLV (Grating Light Valve) device developed by SLM (Silicon Light Machines) as a light strength modulation device for a laser display, that is, light modulator.
In a GLV device 21, as shown in FIG. 13A, a common substrate side electrode 23 of a refractory metal, for example, a tungsten thin film or a nitride film thereof, or of a poly-crystalline silicon thin film is formed on an insulation substrate 22 such as a glass substrate 22 or the like, and a plurality of beams 24, in this example, six beams [241, 242, 243, 244, 245, 246] straddling across the substrate side electrodes 23 in a bridge-like fashion are disposed in parallel. The compositions of the substrate side electrode 23 and beams 24 are the same as those explained in FIG. 11. Namely, as shown in FIG. 13B, a reflective film cum driving side electrode 26 of an Al film of 100 nm or so in thickness is formed on the surface, which is in parallel to the substrate side electrode 23, of a bridge member 25 of a SiN film, for example.
The beam 24 composed of the bridge member 25 and reflective film cum driving side electrode 26 provided thereon is a portion conventionally called a ribbon.
The aluminum film (Al film) used as the reflective film cum driving side electrode 26 is a suitable metal as the material for optical components because of the following reasons: (1) it is a metal that can be comparatively easily formed into a film; (2) the dispersion of reflectance with respect to wavelengths in a visible light range is small; (3) alumina natural oxidation film generated on the surface of the Al film functions as a protective film to protect a reflective surface.
Further, the SiN film (silicon nitride film) composing the bridge member 25 is formed by the use of the low-pressure CVD method, and the SiN film is selected by reason of the physical values of its strength, elasticity constant, and the like being suitable for mechanically driving the bridge member 25.
When a small voltage is applied between the substrate side electrode 23 and reflective film cum driving side electrode 26, the above-mentioned beam 24 moves closer to the substrate side electrode 23 according to the above-mentioned electrostatic phenomenon, and when the application of the voltage is stopped, the beam 24 moves away from the substrate side electrode 23 and returns to an original position.
The GLV device 21 alternately varies the height of the reflective film cum driving side electrode 26 with the movements of the plurality of beams 24 such as moving closer to or moving away from the substrate side electrode 23 (that is, those movements of every other beams), and modulates the strength of light reflected on the driving side electrode 26 by means of the diffraction of light (one beam spot is irradiated on the whole of six beams 24).
Mechanical characteristics of the beam driven by taking advantage of electrostatic attraction force and electrostatic repulsion force are almost predicated on the physical properties of the SiN film formed by the use of the CVD method or the like, with an Al film mainly functioning as a mirror.
By the way, as described above, the substrate side electrode in the MEMS device is formed on an insulation layer of a semiconductor substrate made of silicon, GaAs, or the like, or an insulative substrate such as a glass substrate. As for materials of the electrode, a polycrystalline silicon film or metal film, in which impurities are doped is used. However, since these materials have a crystalline structure, there occurs unevenness on the surface thereof. For example, in the case of a polycrystalline silicon electrode, according to an analysis by AFM (an atomic force microscope), controlling relative roughness RMS (root mean square) value of a surface can be achieved by strictly carrying out temperature control in the manufacturing process, and it is a well known fact that there easily occurs surface relative roughness of 20 nm or more after a conventional film forming process and a semiconductor manufacturing process that have so far been practiced. The degree of the roughness depends on materials and film forming methods as well.
This surface unevenness poses not so serious a problem in terms of the electric characteristics as well as the operating characteristics of the MEMS device, though it often has become problems in the manufacturing process of an optical MEMS device. Namely, the substrate side electrode of the above-mentioned MEMS device is usually positioned under the reflective film cum driving side electrode. In this case, surface unevenness of a lower layer film becomes sequentially transcribed to an upper layer film in the manufacturing process, thereby resulting in the forming of a driving side electrode with piled-up transcribed surface unevenness, that is, the forming of a reflective film therewith on the uppermost layer that is an optically important film surface.
As one of the manufacturing methods of the MEMS device, there is a method in which a multi-layer structure is formed by repeatedly laminating and processing thin films, and thereafter by selectively removing one layer of the multi-layer structure to manufacture a so-called hollow structure that has a void between a substrate side electrode and beam. This manufacturing method is shown in FIGS. 14A to 14D. This example is the case of being applied to manufacturing the above-mentioned MEMS device 1 shown in FIG. 11.
First, as shown in FIG. 14A, the substrate side electrode 3 of, for example, a polycrystalline silicon film is formed on a substrate 2 in which an insulation film 10 such as SiO2 film or the like is formed on the upper surface of, for example, a silicon substrate 9, and after forming a support part 7, a sacrificial layer 18 for forming a void is formed on a surface including the substrate side electrode 3. Next, as shown in FIG. 14B, for example, a silicon nitride (SiN) film 5 and a driving side electrode material layer of, for example, an aluminum (Al) film 4′ constituting a beam are formed on the support part 7 and sacrificial layer 18. Next, as shown in FIG. 14C, the silicon nitride film 5 and aluminum film 4′ are subjected to patterning through a resist mask 19 to thereby form a beam 6 composed of the silicon nitride film 5 and a driving side electrode 4 made of aluminum. Thereafter, as shown in FIG. 14D, by removing the sacrificial layer 18 to form a void 8 between the substrate side electrode 3 and beam 6, the MEMS device 1 is manufactured.
Silicon (for example, non-crystalline silicon, polycrystalline silicon, or the like) or a silicon oxide film is used to form the sacrificial layer 18. When the sacrificial layer 18 is made of silicon, it can be removed by, for example, a mixture of nitric acid and fluoric acid, or by gas etching employing gas which contains fluorine (F). And when the sacrificial layer 18 is made of an oxidized layer, it is conventionally removed by an oxygen fluoride solution, or by plasma etching employing fluorinated carbon gas.
With such optical MEMS device as manufactured to have a three-layer structure of a substrate side electrode (a), a sacrificial layer (b) for forming a void, and a reflective film cum driving side electrode (c), assuming that the maximum values of surface unevenness that are observed in each of the respective layers are Rmax (a), Rmax (b), Rmax (c), there is a possibility that when the three layers are laminated, the amount of surface unevenness on the surface of the uppermost layer adds up to the sum of these maximum values.
Describing the performance of optical components of the MEMS device in which aluminum (Al) is made to serve as a reflective film, 92% of the reflectance of the Al film may possibly be obtained if the film is a bulk Al film. However, if there is no controlling on the amount of this surface unevenness, the reflectance will deteriorate by more than several percentage points, so that only 85% or so thereof can barely be obtained. In an extreme case, it is observed that the surface appears to be clouded up. With such an optical MEMS device, as shown in FIG. 15, (an enlarged view of the relevant part of a driving portion), for example, when the substrate side electrode 3 is formed of polycrystalline silicon, unevenness on the surface of the polycrystalline silicon film increases and is transcribed onto the surface of the driving side electrode (Al film) 4 composing the beam (an Al/SiN laminated film) 6, resulting in the deterioration of light reflectance of the driving side electrode serving as a mirror.
Further, there remains a design problem. A MEMS transducer, that is, the resonant frequency of a beam is usually designed by taking account of the mass of resonance, the tensile force of films in respective regions that support the driving part and the like, though in the present circumstances at a time of designing the values of physicality of the respective films are conventionally computed and designed by using the values of physicality on the assumption that those films are in an ideal thin state. Then, as shown in FIG. 16, for example, in case there exists a semi-sphere of 0.3 μm in the substrate side electrode 3, when the sacrificial layer 18 of 0.5 μm is formed on the substrate side electrode 3, there is formed a semi-sphere b of 1.3 μm in diameter by isotropic film forming; and when the beam 6 is formed thereupon, the surface unevenness of the beam 6 further increases.
When the beam is sufficiently thick in comparison with this 1.3 μm, the unevenness is observed as that inherent in the beam 6. However, when the film thickness of the beam becomes thinner, the own shape of the beam 6 is transformed, and the beam 6 is observed to have, for example, a folded structure (referring to FIG. 17). At this time, there occurs the problem that the MEMS device is unable to have dynamic characteristics in accordance with design. FIG. 18 shows an example thereof. In the case where the tensile force of the beam 6 is taken advantage of to drive the MEMS device, if the beam 6 having the film shape is pulled by both ends using the tensile force, its accordion structure stretches out, resulting in wild fluctuations of the physicality value that is approximated by a spring.
As explained above, the surface unevenness of the substrate side electrode has not only affected the relative roughness of the surface of the beam, but also been a factor in the fluctuations of parameters inherent in the MEMS device such as resonance frequency and the like.